capture url variable across wordpress blog and echo the id

Project Description:
I need someone to capture a variable across a blog

example: www.blog.com/blog-post?id=backdoor

now the variable is ?id=

so in blog post wherever a special token is placed it will echo {backdoor}

If the user goes to a different page the same token will be used.

If user comes onto blog and has no variable then it will replace and url's with a specificed url instead.

This project will pay $50.00 because it's very simple